[R] Data.frames can not hold objects...What can be done in the following scenario?

[Onur Uncu]

Rui and the R-help team,

In Rui's helpful answer below, the function returns a list as output.
When we apply() the function to the data.frame, dataframe$newcolumn
has 2 layers of list before we can access each vector elements. For
instance, dataframe$newcolumn[[1]][[1]] is a vector whereas
dataframe$newcolumn and dataframe$newcolumn[[1]] are lists. Is there a
solution that involves less layers of lists? I am just trying to
understand the R language better.

Thank you.


On Sun, Jun 10, 2012 at 3:18 PM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
> Hello,
>
> What you need is to have your function return a list, not a vector. Like
> this
>
> testfun <- function (x, y) list(seq(x, y, 1))
>
> testframe<-data.frame(xvalues=c(2,3),yvalues=c(4,5))
>
> testframe$newcolumn <- apply(testframe, 1, function(x) testfun(x[1], x[2]))
> class(testframe$newcolumn)  # [1] "list"
>
> Then you access lists and list elements.
>
> testframe$newcolumn[[1]]  # a list with just one element
> testframe$newcolumn[[1]][[1]]  # that element, a vector
> testframe$newcolumn[[1]][[1]][2]  # the vector's 2nd element
>
>
> Since you want the function to return vectors in order to do further
> computations, you'll access those vectors by varying the list index,
>
>
> testframe$newcolumn[[1]][[1]]  # first list, it's only vector
> testframe$newcolumn[[2]][[1]]  # second list, it's only vector
>
>
> Etc.
>
> Hope this helps,
>
> Rui Barradas
>
> Em 10-06-2012 12:29, Onur Uncu escreveu:
>>
>> Thank you Duncan. A follow-up question is, how can I achieve the
>> desired result in the earlier email? (i.e. Add the resulting vectors
>> as a new column to the existing data.frame?)   I tried the following:
>>
>> testframe$newcolumn<-apply(testframe,1,function(x)testfun(x[1],x[2]))
>>
>> but I am getting the following error:
>>
>> Error in `$<-.data.frame`(`*tmp*`, "vecss", value = c(2, 3, 4, 3, 4, 5
>> : replacement has 3 rows, data has 2
>>
>> Thanks for the help.
>>
>>
>> On Sun, Jun 10, 2012 at 12:02 PM, Duncan Murdoch
>> <murdoch.duncan at gmail.com> wrote:
>>>
>>> On 12-06-10 6:41 AM, Onur Uncu wrote:
>>>>
>>>>
>>>> R-Help community,
>>>>
>>>> I understand that data.frames can hold elements of type double, string
>>>> etc but NOT objects (such as a matrix etc).
>>>
>>>
>>>
>>> That is incorrect.  Dataframes can hold list vectors.  For example:
>>>
>>> A <- data.frame(x = 1:3)
>>> A$y <- list(matrix(1, 2,2), matrix(2, 3,3), matrix(3,4,4))
>>>
>>> A[1,2] will now extract the 2x2 matrix, A[2,2] will extract the 3x3, etc.
>>>
>>> Duncan Murdoch
>>>
>>> This is not convenient for
>>>>
>>>>
>>>> me in the following situation. I have a function that takes 2 inputs
>>>> and returns a vector:
>>>>
>>>> testfun<- function (x,y) seq(x,y,1)
>>>>
>>>> I have a data.frame defined as follows:
>>>>
>>>> testframe<-data.frame(xvalues=c(2,3),yvalues=c(4,5))
>>>>
>>>> I would like to apply testfun to every row of testframe and then
>>>> create a new column in the data.frame which holds the returned vectors
>>>> as objects. Why do I want this? Because the returned vectors are an
>>>> intermediate step towards further calculations. It would be great to
>>>> keep adding new columns to the data.frame with the intermediate
>>>> objects. But this is not possible since data.frames can not hold
>>>> objects as elements. What do you suggest as an elegant solution in
>>>> this scenario? Thank you for any help!
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> I would love to hear if forum
>>>>
>>>> ______________________________________________
>>>> R-help at r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-help
>>>> PLEASE do read the posting guide
>>>> http://www.R-project.org/posting-guide.html
>>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>>>
>>>
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>